NdFeB system sintered magnets were discovered in 1982 by Sagawa (one of the present inventors) and other researchers. The magnets have the characteristic that many of their magnetic characteristics (e.g. residual magnetic flux density) are far better than those of other conventional permanent magnets. Hence, NdFeB system sintered magnets are used in a variety of products, such as driving motors for hybrid or electric cars, battery-assisted bicycle motors, industrial motors, voice coil motors used in hard disk drives and other apparatuses, high-grade speakers, headphones, and permanent magnetic resonance imaging systems.
Initial versions of the NdFeB system sintered magnet had the problem that its coercive force HcJ was comparatively low among the various magnetic characteristics. Methods for solving this problem have been commonly known, such as: (1) a method in which a heavy rare-earth element RH (e.g. Dy and/or Tb) is added to an alloy material to improve the crystalline magnetic anisotropy of the main phase; (2) a method in which two kinds of starting alloy powder, including a main phase alloy which does not contain RH and a grain boundary phase alloy to which RH is added, are mixed together and sintered (binary alloy blending technique); and (3) a method in which the size of the individual crystal grains forming the NdFeB system sintered magnet is reduced.
Among these examples, method (3) has the advantage that the coercive force HcJ can be enhanced without lowering the residual magnetic flux density Br. Although its mechanism has not been completely solved, a qualitative interpretation of this phenomenon is that reducing the grain size decreases the number of crystal defects which serve as the sites for reverse magnetic domains to occur in a region near the grain boundaries.
To decrease the size of the crystal grains, it is necessary to reduce the particle size of the alloy powder at the stage of the alloy powder as the material of the sintered magnet. However, as the particle size decreases, the total surface area of the particles of the alloy powder increases, so that the powder becomes easier to be oxidized. In particular, NdFeB system alloy is extremely reactive with oxygen and may possibly ignite. Accordingly, for the reduction of the particle size of the alloy powder, it is necessary to take sufficient countermeasures against oxidization in the material handling and subsequent processes.
Meanwhile, Patent Literature 1 discloses a method in which alloy powder is put in a container and subjected to a magnetic orientation process without being pressed (the so-called “press-less method”). The press-less method is characterized in that the individual particles of the alloy powder are comparatively free to rotate during the magnetic orientation process, which produces the effect of improving the degree of orientation and thereby enhancing the residual magnetic flux density of the eventually created magnet.
In the press-less method, since there is no need to use a large-sized press or similar machine in the magnetic orientation and other magnet production processes, it is easy to perform the entire process in a specific atmosphere (e.g. oxygen-free atmosphere). Actually, such a process is disclosed in Patent Literature 1, whereby the size of the crystal grains can be reduced and the influence of the oxidization can be prevented, so that a NdFeB system sintered magnet with a high level of coercive force HcJ can be created.